1. Field of the Invention
The present invention relates to an apparatus and method for handover of a Mobile Station (MS) in a wireless communication system. More particularly, the present invention relates to an apparatus and method for controlling the transmission power of an MS depending on a channel environment and improving a handover success rate.
2. Description of the Related Art
Generally, portable terminals supporting 2nd Generation (2G) communication systems and 3rd Generation (3G) communication systems perform handover within the same communication system or from one communication system to another. For example, portable terminals supporting Global System for Mobile communication (GSM) systems and Wideband Code Division Multiple Access (WCDMA) systems may perform handover between the GSM systems and may perform handover from a WCDMA system to a GSM system.
FIG. 1 illustrates a signal flow during a handover from a GSM or WCDMA system to a GSM system according to the conventional art.
Referring to FIG. 1, a Mobile Station (MS) 100 periodically reports information of a serving cell and an adjacent cell to a network 110 through step 121 during connection of a voice call. At this time, if a signal level of the serving cell is poor in a state where the MS 100 is accessing a GSM system, as in step 123, the MS 100 reports information in a format that is the same as in a legacy system to the network 110. If a signal level of the serving cell is poor in a state where the MS 100 is accessing a WCDMA system, as in step 125, the MS 100 reports signal information (i.e., an Event 3A) indicating that the serving cell has a value equal to or less than a threshold value, or signal information (i.e., an Event 3C) indicating that the adjacent cell has a value equal to or more than a threshold value, to the network 110. By doing so, in step 127, the MS 100 receives a handover command message from the network 110.
The MS 100 receiving the handover command message transmits a handover access message to the network 110 through steps 129 and 131. Then, the MS 100 starts a preset control timer (i.e., a T3124 timer) and, as in step 133, waits to receive a physical information message before the control timer expires. At this time, if the handover is an intra handover carried out between cells within a Base Station (BS), there is no need to perform a synchronization process. Therefore, the MS 100 directly proceeds to step 135 below without receiving the physical information message.
In step 135, the MS 100 transmits a Set Asynchronous Balanced Mode (SABM) message attempting channel connection to the network 110 and then, waits to receive an Unnumbered Acknowledge (UA), which is a reply to the SABM, from the network 110 before a preset control timer (i.e., a T200 timer) expires. If the UA is not received, the MS 100 retransmits the SABM attempting channel connection through step 137. At this time, the MS 100 transmits the SABM a preset number of times until the UA is received. If, after transmitting the SABM the preset number of times, the UA is not received, the MS 100 returns to a previous serving cell.
If receiving a UA that is a reply message from the network 110 through step 139, the MS 100 transmits a handover complete message to the network 110 and completes a handover procedure through step 141.
As described above, handover is performed if a signal level received by an MS from a serving cell is worse than that of an adjacent cell, and fails if a reply to a handover message transmitted by the MS is not received by the MS within a threshold time period. Also, the MS performs the handover using a predefined output power, i.e., the transmission power corresponding to a power control level received through a Slow Associated Control Channel (SACCH) from a network.
In the case of using a technique for performing handover depending on a Receive Signal Strength Indicator (RSSI) of the MS and reception or non-reception of a reply message as above, there is a problem in that, even if the RSSI of the MS is high, the handover may fail because of the transmission signal quality of the MS. For example, if the transmission signal quality of the MS is poor, handover may fail because the MS fails to transmit the handover access messages of steps 129 and 131 or the SABMs of steps 135 and 137, and thus fails to receive any replies.
Thus, there is a need to provide a handover technique considering the transmission signal quality of the MS.